The present invention relates to a method and equipment for the manufacture of tubular preforms of thermoplastics. The invention particularly relates to equipment that operates according to the so-called "dip blow" process, which is described in the DE-PS No. 1 800 263.
The known dip blow process is carried out so that the preform is formed through the insertion of the blow mandrel by means of relative motion between the mandrel and an outlet nozzle in a chamber filled with softened plastic The subsequent re-emerging of the mandrel carries along the plastics surrounding it.
It has been shown that with this type of equipment no perfectly satisfying method of operation can be achieved, even when the plastics chamber is provided in known manner with an axially slidable plasticizing and conveying screw. Such a screw is advanced on the forward feed of the nozzle-forming sleeve in the plastics chamber in order to obtain an adequate continuity of the plastics flow. Even then the thickness control of the tubular preform is not satisfactory. This is especially true when the preform is to exhibit a particular longitudinal thickness distribution in order to adapt itself to different local forming requirements during a subsequent blow process.
It has been further tried to increase the pressure on the plastics within the plastics chamber by the insertion of a plunger into the plastics chamber in order to bring about a thickness control. But in practice it has consistently been found that a thickness control meeting high requirements cannot be obtained accurately enough by this method. The flow pattern of the plastics in such a plastics chamber is moreover not homogeneous, which fact impairs the method of operation and the quality of the preforms. In the dip blow process, it is necessary to design such a plastics chamber rather long in order to provide sufficient space from the front for the mandrel and from the rear to the plunger. This results in large dimensions of the plastics chamber and leads thereby to a long dwell time of the plastics in the plastics chamber. This is very detrimental in the case of thermally sensitive plastics.
The present invention has as a primary object to improve flow relations and simultaneously to shorten the dwell time of the plastics in the chamber.
These objects are obtained according to the method of the present invention as claimed below.
Additional preferred versions of the method are described and claimed below.
The invention also relates to equipment for carrying out the method of the invention. These preferred embodiments of the invention are characterized individually below.
It is obvious that the equipment described covers only exemplified versions of the method. Thus, naturally, instead of slidable sleeves, the plastics chamber can also have the form of a compressible and expansible bag which carries out the changes in geometric shape of the plastics chamber by the action of an external pressure means or by the creation of partial vacuum in the expansion phase.
The advantage of the present invention resides in the fact that, the change of form of the plastics chamber allows a particularly exact control of the extruded amount of plastics for the tubular preform, without requiring changes in the annular gap of the nozzles.
A particularly advantageous embodiment results when the expansible and contractible plastics chamber consists of essentially only two sleeves; that is, a front and a rear sleeve. It is especially preferred that the rear one, the sleeve facing away from the nozzle, be firmly connected with the screw chamber. A particularly favorable situation results, not only with respect to the flow pattern, but at the same time only a single close sliding fit is needed on relatively small glide surfaces.